A mixer head for such purposes is shown in FIGS. 1 and 2 of U.S. Pat. No. 3,170,638. This mixer head has a mixing chamber comprising two sections in the form of truncated cones; one at each end of a cylindrical middle section which is slotted along its periphery, and a central shaft extends through the mixer head. The conical sections act as centrifugal pumps pumping the substances to be mixed into the cylindrical section, where in a first stage they undergo a hydraulic shear where the two streams meet. The slots in the middle section act in a second stage as specific shear elements, while a third shear stage occurs when the radial discharge from the head meets the slower moving contents of the mixing vessel. The shear forces act to mix the substances and in particular to disperse and dissolve solids in the fluid mixture.
Mixer heads of this type present several disadvantages. Thus, for a given diameter of the mixing chamber and a given rotational speed, the throughput is delimited by the smaller cross-sectional inlet areas of the conical sections. Further, in acting as centrifugal pumps, the conical sections impart to the substances to be mixed a considerable tangential component of velocity, which rather than contributing to the hydraulic shear detracts therefrom. The central shaft extending through the mixing chamber reduces the volume thereof, and thereby the retention time therein for the fluid mixture. Finally, such mixer heads are not immediately accessible for ocular inspection after a cleaning-in-place procedure (CIP-procedure) due to the presence of the conical sections and the thoroughgoing shaft.
Another mixer and disperser head is shown in FIGS. 1-3 of U.S. Pat. No. 4,900,159. In this mixer head, a pair of impellers is clamped to each end of a generally cylindrical mixing chamber by means of a shoulder and a nut on a shaft extending through a bore in a central hub in the mixing chamber. The mixing chamber has a plurality of axially extending slots in its peripheral wall, which is connected to the central hub by means of a radial flange placed in the middle of the mixing chamber and as a partition separating that in two chambers. Also, in this mixer head the central hub and the flange will reduce the volume of the mixing chambers and thereby the retention time therein for the fluid mixture, and the same parts will likewise impart a rotational velocity to the substances to be mixed, i.e., a tangential component of velocity, which will detract from the shear imparted to the fluid mixture when discharged through the elongated slots. The flange or partition prevents that the two streams from the opposite ends of the mixing head meet and thereby undergo a hydraulic shear. This known mixer head is completely unsuited for a CIP-procedure, partly because of the many inaccessible corners therein, where particulate matter or substances with high viscosity or adhesiveness may accumulate, and partly because of the impellers clamped flatly on to the ends of the cylindrical mixing chamber making an ocular inspection of the inner of the mixing head practically impossible. In fact a thorough cleaning of this known mixer and disperser head will necessitate a complete disassembling of the head, separate cleaning of each of its parts, and reassembling thereof again.
From the applicant's own prior patent U.S. Pat. No. 5,407,271 is known a rotary mixer and disperser head, which alleviates most of the above-mentioned disadvantages. This mixer head consists of a shaft to which is connected a mixing chamber that is to be disposed into a vat or the like for dispersing, dissolving or blending of solids liquids or gasses with other liquids. The mixing chamber has secured to its upper and lower ends a plurality of impeller blades that have an end thereof located outside the mixing chamber to direct material into the mixing chamber and out through openings in the side wall of the mixing chamber during the mixing and dispersing thereof. The shaft for rotating the mixing chamber is merely located at one end thereof and does not extend into the mixing chamber, and thus does not impede the mixing action taking place within it. The specific location of the blades, their relationship relative to the mixing chamber, and their configuration provide for a very efficient mixing operation. However, it has been observed that during operations under difficult conditions, where the load has been high, the weldings between the first impeller blades and the peripheral wall of the mixing chamber tend to break.